Padger wrote:

My greenhouse normally gets heated with paraffin as I do not have
electricity wired so far down the garden.

It is basically a non-starter. The only way you could do better than
turning the parafin directly into heat would be to install a very
expensive heat pump system that produced a perma-frost under your lawn
and would always be least effective in the coldest weather.

I wondered about using a small generator to power an electric heater and
also lighting if feasible.
Anyone with any experience of using one? Would be grateful for any pros and
cons and also a model which wont break the bank (already nearly broken with
the demands of the season)

What you can do with a reasonable level of success is buy a couple of
12v lead acid gel cell batteries around the 18Ah rating and a suitable
charger. Then you can run fairly safe low voltage lighting or slightly
more risky fluorescent lamps from that low voltage battery in the
greenhouse.

Treat the batteries carefully and they will last pretty well. You
recharge them off mains electricity in the house

I did the sums once for making it solar powered - the payback time was
longer than the MTBF of the solar panel. Maplin has most of the bits
although Bull Electrical is usually cheaper for the batteries. Wind
power is possible if you are handy at scavenging in scrapyards and DIY.

Regards,
Martin Brown